The present invention relates generally to semiconductor chip wire bonding devices, and similar bonding apparatus, and particularly to a method and apparatus for locating a bond in order to perform automated in-process optical inspection of wire bonding in such a device.
Semiconductor devices, such as integrated circuit chips, are electrically connected to leads on a lead frame by a process known as wire bonding. The wire bonding operation involves bonding a wire to electrically connect pads residing on a die (semiconductor chip) to a lead in a lead frame. Once the chip and lead frame have been wire bonded, they can be packaged in ceramic or plastic to form an integrated circuit device. A post-process inspection step, commonly called the third optical inspection, typically involves locating the position of all bonds on the device, the wire connections and the wire heights using optical means.
Heretofore the third optical inspection has been accomplished only after the device is completely bonded and sent to a separate machine or operator. In the majority of cases, the inspection is done by a human operator using a microscope. This manual method can be time-consuming and costly.
Separate machines are available to perform this step, but this requires another piece of capital equipment in the production line. Additionally, a post-process inspection machine has a more difficult time locating the bond to perform a successful inspection because all the information about the chip that was available during the bonding operation, such as exact pad and frame positions and information about other detail have been lost.
This is further complicated by the fact that most semiconductor chips have a considerable amount of visual detail (such as the images of the circuits themselves) which must be circumvented in analyzing the post-bond image to find the bonds. In post-process inspections, some of this detail can be mistaken for parts of the bonds.
Leads on the lead frame are highly specular. Their specularity further complicates a post process visual inspection, as lighting conditions may be significantly different in a post-process inspection, thus causing differences in the appearance and images of the highly specular leads.
Given this visual complexity, attempting to find the bonds without the pre-bond image as a reference makes it much more likely that it will be difficult to correctly identify the bonds from the surrounding visual detail in a post-process inspection. Frequently, in such situations, the visual detail that is unrelated to the bond, may be misinterpreted as part of the bond in a post-process inspection, giving rise to erroneous acceptance or rejection rates.
Visual imperfections on the pads and leads caused by probe marks, discoloration, or imperfect illumination further complicate these difficulties. These blemishes may be misconstrued as defects in the bonding process, without the information that was available during the bonding operation.
An additional problem encountered in attempting to perform the inspection in-process can be created by the differences caused by bonding itself. Depending on the type of bonding process and equipment used, heating, cooling, movement and other mechanical factors can create alignment problems for images taken before and after the bonding process, thus making it harder to locate the bond.
Finally, the pre and post-bond images can also become mis-registered simply as a result of phase differences occurring when the two images are digitized from their respective analog inputs by the image processor.